The present invention relates generally to hybrid electric vehicles (HEV) and electric vehicles, and specifically to identifying demagnetization of permanent magnets in motors/generators in hybrid electric and electric vehicles.
The need to reduce fossil fuel consumption and emissions in automobiles and other vehicles predominately powered by internal combustion engines (ICEs) is well known. Vehicles powered by electric motors attempt to address these needs. Another alternative known solution is to combine a smaller ICE with electric motors into one vehicle. Such vehicles combine the advantages of an ICE vehicle and an electric vehicle and are typically called hybrid electric vehicles (HEVs). See generally, U.S. Pat. No. 5,343,970 to Severinsky.
The HEV is described in a variety of configurations. In one configuration, the electric motor drives one set of wheels and the ICE drives a different set. Other, more useful, configurations exist. For example, a series hybrid electric vehicle (SHEV) configuration is a vehicle with an engine (most typically an ICE) connected to an electric motor called a generator. The generator, in turn, provides electricity to a battery and another motor, called a traction motor. In the SHEV, the traction motor is the sole source of wheel torque. There is no mechanical connection between the engine and the drive wheels. A parallel hybrid electrical vehicle (PHEV) configuration has an engine (most typically an ICE) and an electric motor that work together in varying degrees to provide the necessary wheel torque to drive the vehicle. Additionally, in the PHEV configuration, the motor can be used as a generator to charge the battery from the power produced by the ICE.
A parallel/series hybrid electric vehicle (PSHEV) has characteristics of both PHEV and SHEV configurations and is sometimes referred to as a xe2x80x9cpowersplitxe2x80x9d configuration. In one of several types of PSHEV configurations, the ICE is mechanically coupled to two electric motors in a planetary gear-set transaxle. A first electric motor, the generator, is connected to a sun gear. The ICE is connected to a carrier. A second electric motor, a traction motor, is connected to a ring (output) gear via additional gearing in a transaxle. Engine torque can power the generator to charge the battery. The generator can also contribute to the necessary wheel (output shaft) torque if the system has a one-way clutch. The traction motor is used to contribute wheel torque and to recover braking energy to charge the battery. In this configuration, the generator can selectively provide a reaction torque that may be used to control engine speed. In fact, the engine, generator motor and traction motor can provide a continuous variable transmission (CVT) effect. Further, the HEV presents an opportunity to better control engine idle speed over conventional vehicles by using the generator to control engine speed.
The generator motor and the traction motor include permanent magnets. These permanent magnets may demagnetize by accident and may degrade or demagnetize over time due to temperature, high current ripples, power ripples, vibration and aging. The demagnetization may degrade vehicle performance such as output power/torque and efficiency. Indeed, the demagnetization may reach a point where safety becomes an issue. More specifically, demagnetization may result in less torque being available to drive the wheels at a critical point, for example, to pass a vehicle. And, demagnetization may result in less energy being available for regenerative braking, which may adversely affect stopping distance/time.
U.S. Pat. No. 5,650,706 issued to Yamada et al. (xe2x80x9cYamadaxe2x80x9d) is directed to a control device for a salient pole type permanent magnet motor. The object of that device is to prevent torque from lowering due to demagnetization of the magnet. A magnetic flux of the permanent magnet is calculated or inferred by determining an electromotive force of the permanent magnet in accordance with a voltage and current supplied to the permanent magnet motor, a rotational speed of the motor, and an inductance of the permanent magnet motor. This electromotive force is compared to a reference electromotive force representative of a fully magnetized permanent magnet. This process is complex and cumbersome. Direct detection of demagnetization is suggested in Yamada by using certain sensors, such as a Hall device or a magnetoresistance element. These direct detection methods suggested in Yamada are relatively expensive and impact serviceability due to location of a complex sensor in the motor housing. Also, demagnetization beyond a safety limit is not monitored and reported for safety-related actions. Furthermore, no specific demagnetized magnets are identified for maintenance.
Therefore, a need exists for an improved method and apparatus for monitoring permanent magnet demagnetization and identifying which magnet(s) within a permanent magnet motor has failed.
Accordingly, an object of the present invention is to provide a detection method for determining the specific location of a degraded (demagnetized) permanent magnet for a motor of an electric or a hybrid electric vehicle (HEV).
Another object of the present invention is to provide a safe and direct method for determining the magnetic flux of a permanent magnet in a motor.
Yet another object of the present invention is to determine a state of magnetism of a permanent magnet to adjust a torque of a motor in a vehicle.
Yet another object of the present invention is to provide adaptive strategies to compensate for permanent magnet demagnetization, including protection of components, limited operation, and notification of permanent magnet demagnetization to a user of the vehicle.
Yet still another object of the present invention is to detect and identify demagnetized permanent magnets in a motor in a vehicle.
Other objects of the present invention will become more apparent to persons having ordinary skill in the art to which the present invention pertains from the following description taken in conjunction with the accompanying figures.
In accordance with one aspect of the present invention, a device is provided for detecting permanent magnet demagnetization in a motor in a vehicle. The device includes a voltage monitor that detects a permanent magnet induced voltage within the motor at a predetermined speed and no load condition. The voltage monitor is coupled to a processor that receives the permanent magnet induced voltage, as measured at the predetermined speed, and compares the permanent magnet induced voltage to a reference voltage that reflects the permanent magnet induced voltage for the motor with a fully magnetized permanent magnet. The processor determines a difference in the detected permanent magnet induced voltage and the reference voltage. The difference is analyzed to determine if a permanent magnet is demagnetized. In particular, the permanent magnet induced voltage is a function of the relative positions and locations of the permanent magnets in the motor. This relationship is used to identify a demagnetized magnet. In particular, the permanent magnets are configured such that a change in magnetic reluctance or magnetic strength is used to identify a demagnetized magnet. A diagnostic code is set to alert others of the position of the failing magnet for replacement or other corrective action.
In accordance with another aspect of the present invention, a method is provided for identifying demagnetization of a permanent magnet in a motor of a vehicle. The method includes the step of detecting a first signal that is a function of magnetism of a plurality of permanent magnets in the permanent magnet motor. Then the first signal is compared with a reference signal that represents a function of magnetism of the plurality of magnets in the permanent magnet motor. The reference signal reflects a level of magnetization that is expected where the plurality of permanent magnets in the motor are fully magnetized. A difference between the first signal and the reference signal is analyzed to determine a demagnetized permanent magnet that is likely causing the difference. In particular, the first signal and reference signal include points of synchronization that relate to the position of potentially demagnetized permanent magnets. More specifically, the points of synchronization are caused by a predetermined change in structure of the motor at a particular location relative to the location of the permanent magnets. This change in structure results in a change in motor reluctance or magnet strength that is reflected in the first signal and the reference signal. Hence, differences between the first signal and the reference signal are correlated to a position of a demagnetized permanent magnet. A device in accordance with the invention includes a processor that executes the method described above.